SAN FRANCISCO—In Tuesday’s mayoral election here, 16 candidates will battle in one of the most competitive contests in years. And to influence the race’s outcome, Silicon Valley technology players are getting involved in the election to an unprecedented degree.

In the past several weeks, founders of San Francisco-based Salesforce.com Inc., Twitter Inc., Zynga Inc. and Yelp Inc., along with several investors in those companies, have contributed thousands of dollars to try to keep interim mayor Ed Lee in office. They support Mr. Lee’s policies that have shaved millions of dollars from some Web companies’ tax liabilities in an effort to …

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